Conventionally, it is practice to pulverize an ingot of hydrogen absorbing alloy obtained by melting and filling the resulting powder into a container of specified capacity, for example, for use in batteries wherein the electrode is made of hydrogen absorbing alloy and in heat pumps wherein such alloy is utilized. Hydrogen absorbing alloys need to be treated for activation so as to exhibit their hydrogen absorbing and desorbing ability to the greatest possible extent. The alloy is pulverized, thereby given an increased surface area and rendered easy to activate.
In order to realize desired features of hydrogen equilibrium pressure conforming to the purpose of use of heat pumps or the like and further obtain a hydrogen absorbing agent having a capability of storing a large amount of hydrogen and an increased speed of reaction, it has been proposed to admix (A) 20 to 80 wt. % of Mg--Ni alloy with (B) 20 to 80 wt. % of an alloy selected from the group consisting of Ca--Ni alloys, Mm(misch metal)-Ni alloys and Mm--Co alloys (Examined Japanese Patent Publication 59-46881).
It has also been proposed to admix a small amount of a hydrogen absorbing alloy of the titanium-zirconium-manganese type having the drawback of marked hystresis and small heat of reaction with a misch metal-nickel-aluminum alloy having the drawback of being relatively small in the amount of hydrogen to be absorbed per unit weight and low in reaction velocity (Examined Japanese Patent Publication SHO 61-40021).
When a powder of hydrogen absorbing alloy is to be filled into a container of definite capacity, it is desired to fill a large amount of the alloy with voids diminished to the greatest possible extent. However, the particles of hydrogen absorbing alloy obtained by pulverizing an ingot differ variously in size and shape and are difficult to fill to a high density with voids inevitably formed among the particles.
In the case where the container of definite capacity is to be packed with particles of approximately the same size, it is difficult to make the void fraction less than 0.5 which is the limit value for the void fraction to be involved when the particles are assumed to be spherical, because the particles are actually not spherical. The term "void fraction" as used herein means the ratio of the voids (spaces) to the bulk volume of a material.
On the other hand, if powders which are different in average particle size are mixed together and filled, small particles will enter interstices between large particles to result in a decreased void fraction.
Accordingly, in packing a container with powders of hydrogen absorbing alloy which are different in particle size, shape, etc., an object of the present invention is to provide an increased hydrogen absorbing capacity per unit internal volume of the container by filling the powers into the container to a high density so that the resulting void fraction is smaller than 0.5.
The present invention gives an increased hydrogen absorbing capacity to the energy storage container of fuel cells and a higher capacity to cells wherein the electrode is made of hydrogen absorbing alloy and provides compacted heat pumps wherein hydrogen absorbing alloy is utilized.